Many recomandations I have seen (and follow) is to go about 1 pound per gallon (so 10 pounds of gravel). Personally, I don't like gravel, and certainly not bright colors. After having gravel in both of my tanks originally I have since switched to sand. And tank I get in the future will have a sand substrate. I recommend sand. Of course, what type of gravel you use is almost completely up to you. It's your tank, use something you like.
But do keep in mind your fish, for example, if you want cories you should get sand or a curved gravel without sharp edges, as the sharp edges can damage cories barbels and lead to infection. Mostly it depends on what you like. Most fish are not overly picky about their gravel, but some are, so do keep that in mind. Also, a darker substrate more often than not leads to much more brightly colored fish.
EDIT: Almost forgot, if you are like me, not likely to run out and buy stockings
you can also use a small plastic container (new, unused) and keep some of your current seeded gravel in the container in the tank to help your cycle.
\Dan